This invention pertains to the radiographic arts and more particularly to automatic exposure controls for radiographic diagnostic apparatus. The invention is particularly applicable to radiographic apparatus for producing video encoded shadowgraphs of a region of the patient. More specifically, it is applicable to an apparatus which processes a plurality of video encoded shadowgraphs by superimposition and other mathematical techniques to produce electronically enhanced video shadowgraphic images of a patient. It will be appreciated, however, that the invention has broader applications in other radiographic apparatus which produce an intermediate optical image.
In the past, others have devised radiographic apparatus for producing shadowgraphs which are displayed on a video monitor. Some of these systems are shown by way of example in U.S. Pat. No. 3,573,461, issued Apr. 6, 1971 to S. A. Ohlsson, U.S. Pat. No. 3,784,816, issued Jan. 8, 1974 to S. Abrahamsson, or U.S. Pat. No. 3,848,130, issued Nov. 12, 1974 to A. Macovski. Such systems consist of an x-ray source for irradiating the patient or other object to be examined and a fluorescent screen for converting the x-radiation into an optical image. A television camera which is disposed to view the optical image produces a video representation of the image. The video representations are manipulated by a computer or processor and displayed on a video monitor.
To produce a good video image, the television camera must receive sufficient light from the optical image to produce good contrast, but no so much light that the image is washed out. Further, it is undesirable to irradiate the patient with radiation for a longer duration than is necessary or with a higher intensity than is necessary. Three factors have been varied to obtain a properly exposed video image--adjusting the KV of the power supply which determines the penetrating power of the x-rays, adjusting the milliamperes (Ma) of the power supply which determines the intensity of the x-rays, and adjusting the duration that the x-ray source is actuated. These determine the brightness of the optical image and the duration which the optical image is available for the television camera to monitor.
The intensity of the x-rays reaching the fluorescent screen, is greatly effected by the thickness and density of the patient or object through which the radiation has traversed. Small variations in the thickness, density, or other radiation absorptive properties of the patient produce relatively large differences in the intensity of radiation reaching the fluorescent screen. Accordingly, each time a new patient or different part of the same patient is to be examined, it is necessary to redetermine the optimum exposure. In the past the optimum exposure was determined by trial and error. That is, the operator would adjust the milliamperes and the duration and possibly the KV to values which he felt would produce a good image. After examining the produced image, the operator would readjust milliamperes or duration to improve the quality of the image produced. This trial and error procedure would often result in x-raying the patient a half a dozen times merely to calibrate the exposure without producing usable data. The radiation exposure of the patient during calibration is undesirable and excessive.